Buy and hold will work until it doesn't and a lot of people will be left holding the bag, especially if you need the money at the bottom for medical, retirement, etc...
Just check the Japan’s Nikkei stock index, almost 30 years later people are still waiting and are far far away from the top...
So what you're saying is that if the market starts behaving significantly different than it has throughout history, people will be screwed. How exactly does backtesting help you in this scenario? By definition, basing a strategy off a good backtest is assuming that the market will behave similarly in the future as it has in the past.
People are people, they are greedy, selfish and they scare all the same no mater if it was the tulip or the dot com or the financial bubbles, that's why I am confident in my back tests and that's why I am investing and trading successfully and I am happily waiting for the next bubble to shake things up and present even more opportunities...
At least, it's worked reasonably well for me for the last 20 years.